Bulletin of the Geological Society of Malaysia, Volume 22, Dec. 1988; pp. 1 – 22
Texaco Canada Resources Ltd., Calgary. Alberta, Canada
Abstract: Carbonate diagenesis plays an important role in the exploration for, and hydrocarbon recovery from, pinnacle reefs of the Upper Devonian Nisku Formation of Alberta.
In the West Pembina area, the Nisku Formation occurs as a series of carbonate ramps prograding into the Winterburn shale basin. Discovered on a regional seismic line in 1977 by Chevron Standard Ltd., the play has matured into a pinnacle reef fairway 65 km by 180 km containing over 50 productive reefs with total reserves of approximately 325 MMstb oil and 500 BCF gas. Texaco‘s discovery, alone or jointly of 27 of these reef pools for a 30% net share in reserves has made understanding reservoir quality of importance to maximizing reserves on the reef trend.
Carbonate diagenesis exerts a major influence on the development of porosity and permeability in the reefs, and accounts for areal variability among different reefs in the trend. On an exploration basis, knowledge of diagenesis highlights areas for preferred drilling. For development, diagenesis and its effects upon reef petrophysics affects the selection of pools for water versus miscible flood enhanced recovery schemes.
The Nisku pinnacle reefs are built by rugose corals, stromatoporoids and algae with interbedded and matrix lime mudstones and wackestones. Three diagenetic processes affecting porosity and permeability within the reefs are early submarine cementation, dolomitization, and pressure solution.
The early submarine cements include spectacular multiple fibrous cements together with stromatolitic crusts and micritic cements. While these cements occluded some primary porosity, they provided a net beneficial effect by stabilizing the reef framework and muddy matrix and preserving intergranular and large interfossil pores.
Dolomite is the major determinant of reservoir quality. Fully dolomitized reefs average 10-20% porosity and up to 5 darcies permeability whereas partly dolomitized reefs have porosities of 2-5% and approximately 500 millidarcies permeability. Two dolomite types are present and both are believed to result from burial processes. Microdolomite rhombs 10 to 200 µm in size selectively replace micrite matrix in all reefs and are considered a shallow burial product which increases matrix permeability somewhat. Coarse sucrosic dolomite, with crystals from 50 to 400 µm in size, is not fabric selective and pervasively obliterates primary rock textures. This dolomite is a deep burial product which greatly increases intercrystalline porosity and permeability of the reefs, and accounts for reservoir variations along the reef trend.
Pressure solution has occurred throughout the burial history of the reefs. It is most noticeable as high amplitude stylolites concentrated along facies contacts and testifies to significant removal of potential reservoir section by solution. Additional leaching of skeletal fragments has created secondary porosity.
Understanding the interplay of these diagenetic processes upon the evolution of reservoir quality may have application to reef plays of other areas or geologic age.